Date of Award
Master of Science (MS)
Charles R. Stern
Detrital zircon U-Pb ages are commonly used to assess the provenance of siliciclastic sediments. However, in Cambrian siliciclastic units in North America, such determinations are limited by the wide spatial distribution of Proterozoic crystalline basement rocks with similar U-Pb ages. In this study, combined U-Pb and Hf isotopic data were obtained from 1.0 to 1.3 Ga, so called “Grenvillian”, detrital zircons in sedimentary rocks from central North America, in order to better define the geographic source locations and sediment transport directions after the breakup of the Mesoproterozoic supercontinent Rodinia. The whole-rock radiogenic isotopic compositions of granitic rocks from a 1.086 ± 0.02 Ga anorthosite-charnockite-mangerite-granite (AMCG) suite in Sonora, Mexico were used to identify the petrogenetic nature of these Grenville-age crustal intrusions. Additionally, Hf isotopic compositions were determined for zircon from this suite of samples to help complete the existing Hf isotope database for potential sources of Grenvillian detrital zircon. All samples have low measured εHf values ranging from -25.5 to -21.1 and are determined to have crystallized from a melt that incorporated a large component of Paleoproterozoic crust of the Yavapai/Mazatzal Province that underlies portions of northwestern Mexico. These data and existing isotopic data for other 1.0 Ga to 1.3 Ga rocks throughout North America were applied to determine the provenance of Grenvillian detrital zircon found in four Neoproterozoic and Cambrian basal sandstone units from the central U.S. Three of these samples were are from fluvial to shallow marine depositional environments, and are interpreted to be deposited after the breakup of Rodinia during the transgression of the early Paleozoic Sauk Sea; the fourth sample is a Neoproterozoic injectite sandstone that is interpreted as a pre-Sauk Sea sedimentary unit. The easternmost sample, the Cambrian Lamotte Sandstone in southeastern Missouri, is observed to contain a major zircon population of ~1.1 Ga with a narrow range of Hf isotopic compositions that range ¬from εHf = -27.4 to -20.0. Samples from basal sandstones from Colorado drill cores have a broad Grenvillian age peak from 1.0 – 1.3 Ga, accompanied by peaks at ~1.4 and ~1.7 Ga. The Grenvillian detrital zircon in these samples exhibit a wide range of εHf that falls in the wide range of values also observe in the nearby Neoproterozoic Tava Sandstone Injectites (εHf = -42.5 to -14.4). The Lamotte Sandstone in the east potentially originated from a limited catchment area in the Southern Appalachian Mountains, while western basal sandstones incorporated zircon from multiple sources, such as the Southern Appalachians, the Adirondacks, the Llano Uplift, the Franklin Mountains, and local felsic intrusions. Interestingly, Flathead sandstone from central Wyoming is dominated by ~1.7 Ga zircons with a distinct absence of Grenvillian zircon, indicating that the supply of Grenvillian zircon from the south and east had been shut off to this region. Broadly, the observed increase in range of U-Pb and εHf values with westward geographic position is interpreted to be the result of the mixing of multiple zircon sources with westward sediment transport. The presence or absence of Grenvillian detrital zircon in basal sandstones provides information of the relative timing and paleogeography of Laurentia during the Cambrian. The absence of Grenvillian zircon is unique in these sandstones, and requires some type of sediment transport barrier, such as a topographic structural high of the proposed Paleozoic Transcontinental Arch, or a potential oceanic barrier, such as the Sauk Sea. These observation will require additional detrital zircon data from other North American basal sandstones to confirm and establish the sedimentation controls of Laurentian cross-continent zircon transport.
Hantsche, Aaron L., "Hafnium Isotope Evidence on the Source of Grenvillian Detrital Zircon Deposited at the Great Unconformity" (2015). Geological Sciences Graduate Theses & Dissertations. 106.